[PATCH 8/8] rwsem: small optimizing rwsem_down_failed_common

[Alex Shi]

Peter Zijlstra suggest adding a preempt_enable_no_resched() to prevent
a unnecessary scheduler in raw_spin_unlock.
And we also can pack 2 raw_spin_lock to save one. So has this patch.

Thanks Peter!

Signed-off-by: Alex Shi <alex.shi@xxxxxxxxx>
---
 lib/rwsem.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/lib/rwsem.c b/lib/rwsem.c
index ad5e0df..9aacf81 100644
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -212,23 +212,25 @@ rwsem_down_failed_common(struct rw_semaphore *sem,
 		 adjustment == -RWSEM_ACTIVE_WRITE_BIAS)
 		sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
 
-	raw_spin_unlock_irq(&sem->wait_lock);
-
 	/* wait to be given the lock */
 	for (;;) {
-		if (!waiter.task)
+		if (!waiter.task) {
+			raw_spin_unlock_irq(&sem->wait_lock);
 			break;
+		}
 
-		raw_spin_lock_irq(&sem->wait_lock);
-		/* Try to get the writer sem, may steal from the head writer: */
+		/* Try to get the writer sem, may steal from the head writer */
 		if (flags == RWSEM_WAITING_FOR_WRITE)
 			if (try_get_writer_sem(sem, &waiter)) {
 				raw_spin_unlock_irq(&sem->wait_lock);
 				return sem;
 			}
+		preempt_disable();
 		raw_spin_unlock_irq(&sem->wait_lock);
+		preempt_enable_no_resched();
 		schedule();
 		set_task_state(tsk, TASK_UNINTERRUPTIBLE);
+		raw_spin_lock_irq(&sem->wait_lock);
 	}
 
 	tsk->state = TASK_RUNNING;
-- 
1.7.12

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